A general atmospheric pressure ionization mass spectrometer introduces ions generated under atmospheric pressure into vacuum and analyzes mass of the ion.
An ion source generating ions under atmospheric pressure includes various methods, such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and matrix assisted laser desorption/ionization (MALDI). However, materials, which becomes noise components other than desirable ions, are generated in any of the methods. For example, in the ESI ion source, while a sample solution is flowed in a metal capillary with a small diameter, a high voltage is applied thereto to ionize the sample. Accordingly, noise components other than the ion, such as charged droplets or neutral droplets, are simultaneously generated.
The general mass spectrometer is divided into several spaces respectively divided by apertures, and each space is exhausted by a vacuum pump. As it goes to a rear stage, degree of vacuum is higher (pressure is lower). A first space divided from atmospheric pressure by a first aperture electrode (AP1) is exhausted by a rotary pump or the like and often held at degree of vacuum of about several hundred Pa. A second space divided from the first space by a second aperture electrode (AP2) has an ion transport unit (a quadrupole electrode, an electrostatic lens electrode, and the like), which transports ions while focusing it, and is often exhausted at about several Pa by a turbomolecular pump or the like. A third space divided from the second space by a third aperture electrode (AP3) includes an ion analysis unit (an ion trap, a quadrupole mass filter, a collision cell, time-of-flight mass spectrometer (TOF), and the like), which performs separation or dissociation of ions, and a detection unit detecting ions. The third space is often exhausted at 0.1 Pa or less by the turbomolecular pump or the like. There is also a mass spectrometer divided into more than three spaces, but a device consisting of about three spaces is generally used.
The generated ions (including a noise component) pass through the AP1 and are introduced into a vacuum chamber. After that, ions pass through the AP2 and are focused on a central axis in the ion transport unit. After that, ions pass through the AP3, and are separated at every mass or dissociated in the ion analysis unit. Accordingly, a structure of the ion can be analyzed in more detail. Eventually, ions are detected by the detection unit.
In the most general mass spectrometer, the AP1, AP2, and AP3 are often disposed coaxially. Since the aforementioned droplet other than the ion is hardly affected by an electric field of the aperture electrode, the transport unit, or the analysis unit, it basically tends to go straight. Because of that, there is a case where a surface or the like of each aperture electrode having a very small diameter is contaminated.
Therefore, in the general mass spectrometer, it becomes necessary to remove and clean the AP1 or the AP2 periodically. However, a vacuum system, such as a vacuum exhaust pump, needs to be stopped for the cleaning, and it generally takes one day or more to stably operate the vacuum system after restarting it. Further, excessive introduction of the droplets, which goes straight, may reach the detector and also leads to shorten a life of the detector.
In order to solve this problem, in PTL 1, a member having a plurality of holes is disposed between an ion source and an AP1. Since no hole is opened in this member at a position coaxial with the AP1, introduction of noise components from the AP1 can be reduced. However, since this member having a plurality of holes is disposed outside the AP1, both front and rear sides of this member are in a state of atmospheric pressure.
On the other hand, in PTL 2 or PTL 3, droplets, which goes straight, are removed by orthogonally disposing an axis of an AP1 outlet and an axis of an AP2. However, a space between the AP1 and the AP2 bent at a right angle is exhausted by a vacuum exhaust pump, such as a rotary pump, in a direction orthogonal to the axis of the AP2.